Low-pressure discharge lamps are well known in the art. These lamps contain small doses of mercury, which radiates in the short range UV spectrum under the influence of a discharge arc. In order to achieve maximum luminous output or luminance, it is required that the mercury vapor is adjusted and stabilized at a well-defined partial pressure. This is possible by forming a so-called cold chamber on the discharge tube, and by selecting the appropriate temperature in the cold chamber, which is the coldest point of the gas discharge tube.
German Patent No. DE 35 44 465 discloses a U-shaped compact fluorescent lamp with two straight tube members and a connecting middle portion and a method for production. A cold chamber is defined by a longitudinal extension of the straight tube members beyond the connecting middle portion. The longitudinal extension is selected in the range of 0.16*D to 0.6*D, wherein D represents the diameter of the straight tube member. In this configuration, the cold chamber is positioned at an the end of the tube opposite to the electrodes. In vertical operation of the lamp, this will be the uppermost section when used in an upright or base-down position or it will be the lowest section of the lamp when used in a downward or base-up position, e.g. in a ceiling lighting. Such a cold chamber configuration will not provide the same effect when using the lamp in the upright or downward position. In the upright position, the temperature of the cold chamber will be significantly higher than in the downward position resulting in a lower luminous output.
German Patent No. DE 41 33 077 discloses a helical shaped discharge lamp, with a double helix discharge tube. In this known discharge lamp, the cold chamber is positioned at the top of the lamp, between the two ends of the tube portions constituting the strands of the double spiral. An annular widening of the discharge tube forms the cold chamber. However, the light distribution of the lamp in the region of the cold chamber still needs improvement, because a relative large portion of the enveloping surface is not utilized as lighting surface, particularly in the direction along the axis of the lamp, towards the end which is further away from the lamp housing. This is particularly configured so that is suitable for screwing in a socket on the ceiling, with the cold chamber facing downwards. Again, the lamp does not provide the same luminous output if it operates in a base-down position. The luminous efficacy of the lamp may be smaller by 10% or more.
Therefore, there is a need for a discharge tube or lamp, which exhibits improved luminance, e.g. no difference in the luminous output of the lamp may be perceived when the lamp is operated in different positions. More specifically, there is a need to provide a discharge tube or lamp, which does not exhibit a perceptible decrease in the luminous efficacy when operated in a base-down position instead of a base-up position. There is a further need for a lamp, which does not exhibit a perceptible decrease in the luminous efficacy when operated even in a horizontal position instead of a vertical base-up position. Therefore a discharge tube or lamp configuration is required, which has an efficient cold chamber for optimum performance of the lamp.